Lead levels in Illinois water systems

Because of the health risks posed by lead contamination, the federal government requires water systems across the country to test samples from household taps for the toxic metal. Most water systems in Illinois have found lead at least once in the past decade; the state has more lead pipes connecting homes to water mains than any other. But a water system is not required to take action unless more than 10 percent of the samples taken exceed 15 parts per billion during a full testing cycle, often three years. To examine residents' potential exposure more closely, the Tribune analyzed test results by calendar year and found that more than 170 water systems in Illinois have exceeded the federal lead standard at least once in the last dozen years. Those systems serve hundreds of thousands of people in all. Look up test results for your own water system below.

Water systems where lead testing found high levels

The dots on the map below represent water systems where the Tribune found lead test results exceeded the federal standard in at least one year since 2004. (The Environmental Protection Agency does not consider a water system to be in violation unless test results exceed standards in a full testing cycle, which is often three years.) Scroll over the map to see the names of the water systems and other details.

Lead test results for local water systems

Local water systems typically are not required to notify homeowners about lead levels or take other action unless test results exceed federal standards during a full testing cycle, which may be three years long. To provide a more detailed picture of residents' potential exposure, the Tribune analyzed lead results by calendar year.

The EPA's procedures involve identifying the 90th percentile for the test scores and comparing that figure to the standard of 15 parts per billion. To carry out that calculation, the Tribune used a common mathematical formula that allowed for a consistent comparison. The EPA directs officials to use one of several alternative methods that may produce different results.

Find results for your water system by typing its name in the "Search" input below. In many cases, your water system is the name of your city or town, such as Chicago, Marengo or Galesburg. Years in which the Tribune found the system exceeded the federal standard are highlighted in red.

The Illinois EPA did not provide detailed information for some water systems; many are no longer operating. The EPA says it does not consider scores between 15.1 and 15.4 to be in violation, because it rounds to the nearest whole number for compliance purposes.

Water system

Test year

Lead concentration at 90th percentile (ppb)

Primary county

Number of people served

Number of tests

Understanding the EPA's 90th percentile standard

To evaluate whether a water system meets federal standards for lead, the EPA directs officials to calculate the 90th percentile for all test scores in a given testing cycle. If that figure is higher than the EPA's "action level" of 15 parts per billion, the system is considered to exceed the standard. Another way to look at it is that more than 10 percent of the water system's tests exceeded 15 ppb. The methodology is intended to flag widespread corrosion problems in a community's lead pipes and plumbing, but these rules also can obscure the fact that some test results may be much higher than 15 ppb. The following examples are from the Tribune's analysis, which applied the EPA's standard to individual calendar years instead of full testing cycles.

Above the standard

The Berwyn water system in Cook County recorded 37 test results in 2014. The Tribune calculated the 90th percentile for that year as 22.36 parts per billion. This is above the standard of 15 ppb. If a similar value was found over the course of a full test cycle, often three years, the system would be required to take action. In 2014 the highest individual test result was 198 ppb.

Below the standard

The Chicago water system recorded 51 test results in 2015. The Tribune calculated the 90th percentile for that year as 9.4 parts per billion. This is below the action level of 15 ppb. Chicago operates on a three-year testing cycle and has never reported lead levels that required action, though Tribune reporting has raised questions about its testing locations. In 2015 the highest individual test result was 120 ppb.